While we don't support notes on inquiries, we do support notes on contacts, which are created and available at the inquiry stage, and are also available on the reservation page. It would be an imperfect workaround, but you could always leave your notes on the contact with a message such as "For inquiry on 7/26, renter is interested in a late check-in". The contact notes can be easily accessed and created from the inquiry page by clicking "Renter Info" and clicking the "Notes" tab.
The downside is you would have to make sure to remember to check those notes, as they would not be displayed without some extra clicking around on the inquiry page.
Hopefully this workaround might offer a temporary solution for MyVR users who really need to write notes before reservations are created.